Ceramic structures, usually and preferably multilayered, are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers formed prior to lamination, which, upon sintering, will become a sintered dense metal interconnection of metal-based conductor.
In general, conventional ceramic structures are formed from ceramic greensheets which are prepared by mixing a ceramic particulate, a catalyst (e.g., such as that disclosed in Herron et. al., U.S. Pat. No. 4,627,160), a thermoplastic polymeric binder, plasticizers and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After blanking, stacking and laminating, the greensheets are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate.
The electrical conductors used in formation of the electronic substrate may be high melting point metals such as molybdenum and tungsten or a noble metal such as gold. However, it is more desirable to use a conductor having a low electrical resistance and low cost, such as copper and alloys thereof.
In the area of electronic packaging, there are increasing demands for improvements in performance. Current state-of-the art materials include the cordierite glass ceramic materials disclosed in Kumar et. al. U.S. Pat. No. 4,301,324, the disclosure of which is incorporated by reference herein. These glass ceramic materials have a dielectric constant of about 5.0 and a thermal coefficient of expansion (TCE) of about 30.times.10.sup.-7 /.degree.C. that closely matches that of silicon. It would be desirable to have a substrate material with a dielectric constant of less than 5.0 to reduce signal propagation delay, strength and toughness at least equal to that of the glass ceramics, and a thermal coefficient of expansion close to that of silicon. In order to meet these demands, alternate substrate materials are being investigated.
Fused silica has been proposed for use as a substrate material, since it has a dielectric constant much less than 5.0, but its thermal coefficient of expansion is much less than that of the silicon devices which are placed on the substrate. Significant differences in thermal coefficients of expansion between the substrate material and the silica device lead to thermal stresses in use, thereby making fused silica of limited use as a pure substrate material.
Among the substrate materials being investigated by the present inventors are composites, which might consist of a ceramic matrix plus fibers or whiskers. Silicon nitride whiskers in a glass ceramic matrix are but one example of such a composite material.
Others have investigated composite materials as well.
UK Patent 2 168 338 discloses a molded ceramic body consisting of fibers in a ceramic refractory material. The fibers may be, for example, silica, and the refractory material may be, for example, an aluminosilicate material such as cordierite or a silica material such as quartz. The porosity of the body varied across the ceramic body from 20 to 70%.
Sullivan U.S. Pat. No. 4,962,070 discloses a ceramic body comprised of silica-coated carbon fibers in a cordierite matrix. Generally, the dielectric properties of carbonaceous-based fiber are very poor.
Claussen et. al. U.S. Pat. No. 4,855,259 discloses cordierite bodies containing, for example, silicon nitride, silicon carbide, alumina, magnesium or mullite fibers.
Montierth U.S. Pat. No. 3,715,196 discloses a cementing composition for bonding together glass-ceramic structures. The cementing composition comprises a thermally devitrifiable glass (which upon devitrification may form, e.g., cordierite) and a fiber-flux mixture comprising fused (amorphous) silica fibers.
Yamamura et. al. U.S. Pat. No. 4,610,917 discloses a composite material consisting of fibers containing silicon, either titanium or zirconium, nitrogen and oxygen in a glass ceramic matrix.
Pierson et. al. U.S. Pat. No. 3,940,277 discloses glass ceramic articles in which crystalline silica fibers are formed in situ.
Meyer et. al., "Reinforcing Fused Silica with High Purity Fibers", Ceramic Engineering Science Proc., Vol. 6 [7-8], pp. 646-656 (1985), discloses high purity fused silica fibers dispersed in a high purity fused silica matrix.
Notwithstanding the efforts of those concerned with the present problem, there nevertheless remains a need for a substrate material with a dielectric constant of less than 5.0 and acceptable strength and toughness values.
Accordingly, it is an object of the present invention to have a substrate material with a dielectric constant of less than 5.0 and acceptable strength and toughness values.
It is another object of the present invention to have a substrate material that has a thermal coefficient of expansion close to that of silicon.
These and other objects of the invention will become apparent after referring to the following detailed description of the invention.